1. Technical Field
The invention relates to ceramics technology, especially to a method of making textured ceramics.
2. Description of Related Art
Silicon nitride (Si3N4) ceramics is one of the most promising materials for high-temperature applications. Si3N4 has tough interlocking microstructures consisting of elongated grains. Generally, the elongated grains randomly distribute in Si3N4 ceramics, which result in an isotropic microstructure. However, the rod-like grains in anisotropic Si3N4 are oriented one- or two-dimensionally are expected to show higher flexure strength and fracture toughness in specific directions than those of conventional isotropic Si3N4.
There are three main method to fabricate textured Si3N4. (1) Tap-Casting. Si3N4 with a preferred orientation of large elongated grains was obtained by tape casting of starting powder slurry seeded with rod-like Si3N4.1-4 (2) Hot-forging. The conventional Si3N4 consisting of elongated grains yield a highly anisotropic microstructure by superplastic plane-strain compressive deformation, and the rod-like grains tend to be aligned in the extruding direction.5-6 The sintering-forging was improvement of the hot-forging.7 The texturing mechanism were same to decrease the grain size by reducing the sintering time and temperature, the texturing degree was increased by sintering-forging. (3) Magnetic field alignment. The nonmagnetic ceramic particles are oriented during slurry consolidation under a strong magnetic field (typically ≧10 T), thereby resulting the information of textured ceramics during sintering.8-10
In the three methods mentioned above, the hot-forging is a common method for texturing. As shown in FIG. 1 (a), the hot-forging method were consisted of two steps: sintering and forging.11 First, the β-Si3N4 with high density and isotropic rod-like grains was obtained in high temperature sintering process. Second, the forging was conducted at high temperature with high pressure, the alignment of the elongated grains was obtained due to its high temperature superplastic deformation. This process would cost much more time and energy. The texturing degree of the hot-forging was limited by amount of the superplastic deformation which depends on the grain size of the sintered sample, liquid phase, forging temperature, and pressure. In this report, the fabrication of highly 2-dimensional textured (a,b-axis aligned) Si3N4 was attempted to obtain by one step, named it hot pressing flowing sintering (HPFS).